Method and apparatus for compensating for DOT gain in stochastic printing

ABSTRACT

A method for compensating for dot gain in printing with stochastic screens comprises obtaining continuous tone CMYK files and screening the files at a resolution that is lower than the resolution of an output device that will be used to print the files. The lower resolution stochastically screened files are then expanded to the resolution of the output device, thereby effectively increasing the size of the dots by creating a number of subdots for each dot in the lower resolution stochastically screened files. Subdots within the expanded stochastically screened files can then be selectively removed to compensate for dot gain. In addition, overlapping dots in the screened CMYK files are removed and additional stochastically screened files are created that print a dot of a color of the previously overlapping dots and at the location of the previously overlapping dots to prevent colors from running together when printing on certain media. Finally, the removal of subdots in the expanded CMYK files can be performed to correct the color of the images when the expanded stochastically screened file is printed.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.10/850,352, filed May 19, 2004, which is a continuation of U.S. patentapplication Ser. No. 09/998,591, filed Nov. 16, 2001, now U.S. Pat. No.6,741,735, which is a continuation of International Application No.PCT/IB00/00660, filed May 17, 2000, which designates the United States.U.S. patent application Ser. No. 09/998,591 is also acontinuation-in-part of U.S. patent application Ser. No. 09/314,861,filed May 19, 1999, now U.S. Pat. No. 6,507,666.

FIELD OF THE INVENTION

The present invention relates to printing methods in general, and inparticular to methods for compensating for dot gain without decreasingtonal range or losing detail.

BACKGROUND OF THE INVENTION

One of the most common artifacts that occurs in printing is dot gain.Dot gain occurs when the size of an ink dot that is printed on a mediasuch as paper, fabric, aluminum, plastic, etc. is larger than desired.Dot gain can occur due to the compression of ink as it is pressed ontothe media as well as by the spreading of the ink when absorbed bycertain media such as news print, etc. In addition, dot gain can occuras a result of flaws in output devices using the printing process.

Because smaller dots are affected by dot gain more than larger dots, theartifact is particularly noticeable in stochastic screening whereby allthe dots that comprise the images are the same size and relativelysmall. In the past, most methods of compensating for dot gain involvedthe redistribution and/or removal of a number of dots that are printed.However, the redistribution and/or removal of dots generally reduces thenumber of dots per unit area, thereby causing a corresponding loss ofdetail and diminishing the tonal range in there resulting image.

Given the problems associated with dot gain in stochastic printing,there is a need for a mechanism that can compensate for dot gain withoutremoving detail and/or tonal range from an image.

SUMMARY OF THE INVENTION

To address the problems caused by dot gain in printing that utilizesstochastic screens, the present invention comprises a method of creatingstochastic screens wherein the size of the individual dots in thescreens are modified to compensate for dot gain that occurs during theprinting process.

Continuous tone cyan, magenta, yellow and black (CMYK) images arereceived at a first resolution. A CMYK continuous tone image is appliedto a screening program that creates stochastic CMYK files at the firstresolution. The CMYK files are then expanded to the resolution of anoutput device that will print the image. As a result of the integerexpansion, each dot in the original stochastic screen file will berepresented as multiple subdots in the expanded stochastic screen file.One or more of the multiple subdots that correspond to each dot in theoriginal stochastic screen file are removed to compensate for dot gainthat occurs when the expanded stochastically screened images areprinted. In addition, the removal of some of the subdots aidslinearization by improving the tonal range.

According to another embodiment of the invention, the system determineswhen dots in the original stochastic screen files overlap to create aparticular color. These dots are then removed from the originalstochastically screened files and additional screen files are added thatprint dots with the color and the location of the previously overlappingdots.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a conventional stochastic screeningprocess;

FIG. 2 illustrates a conventional method of compensating for dot gain bythe removal and rearrangement of dots;

FIGS. 3 and 3A illustrate a method in which dots in a lower resolutionstochastic screen file are expanded to create a number of subdots and aportion of the subdots are removed to compensate for dot gain, accordingto the present invention;

FIG. 4 is a flow chart of the steps performed by the present inventionto compensate for dot gain;

FIG. 5 illustrates a method in which overlapping dots in CMYK stochasticscreen files are removed and replaced by additional screens that print adot of a corresponding color at the location of the overlapping dots,according to another aspect of the present invention; and

FIG. 6 illustrates a common stochastic pattern that can be compensatedfor dot gain by removing one or more subdots.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated above, the present invention is a method for compensatingfor dot gain in printing methods that use stochastic screens.

FIG. 1 illustrates a conventional printing system that utilizesstochastic screens. A continuous tone image 10, such as a photograph, isapplied to a scanner 12 that converts the continuous tone image into aseries of RGB values that can be displayed on a computer screen. The RGBvalues are applied to a computer 14 that runs a layout program thatallows an operator to adjust the position of the photograph on a page,add text, etc., and otherwise prepare a document for printing. Thecomputer system also runs a conversion program that converts the RGBvalues into a series of continuous tone grey scale files that representcyan, magenta, yellow and black (CMYK). The continuous tone files foreach color typically have the same resolution as the RGB files producedby the scanner 12.

In order to print the continuous tone CMYK files 16, the CMYK files 16are applied to raster image processor (RIP) and a screening program 18.In conventional printing with stochastic screens, the RIP increases theresolution of the continuous tone files to the resolution of an outputdevice that will be used to produce bit mapped files in the printingprocess. The increased resolution is obtained by scaling up thecontinuous tone CMYK files depending on the resolution of the outputdevice. For example, if a continuous tone CMYK file has a resolution of300×300 pixels per inch and the output device has a resolution of2400×2400 dpi, then the continuous tone CMYK file would grow 64 times tomatch the output device resolution. After increasing the resolution ofthe continuous tone CMYK , the screening program converts the individualCMYK pixels into a stochastic pattern of dots in accordance with theintensity of each of the CMYK pixels. The output of the RIP and thescreening program 18 are a series of bit mapped CMYK stochastic screenfiles 20 having a resolution equal to the resolution of the outputdevice. The high resolution stochastic screen files 20 are then appliedto the output device such as a printer 22 that produces a color image 24from the bit-mapped files.

As indicated above, one of the most typical artifacts that occurs withprinting is dot gain. Dot gain occurs when the size of the ink dotplaced on the media on which the image is to be printed is different(usually larger) than the size of the ink dot requested. For example,when the ink is pressed onto the printing media, the compression causesthe ink to expand. In addition, the ink may soak into certain types ofmedia, such as newsprint, thereby increasing the size of the dotsproduced.

In conventional printing using stochastic screens, dot gain is estimatedby printing a test image, measuring the size of the dots produced with adensitometer and then adjusting the high resolution stochastic CMYKfiles to compensate for any dot gain. Such compensation typicallyinvolves the removal and/or rearrangement of dots from the stochasticscreen files. Because the continuous tone CMYK files are expanded priorto conversion into stochastic screen files, removal of dots from thestochastic screen files causes a corresponding reduction in the level ofdetail and tonal range in the final printed image.

FIG. 2 illustrates a conventional method of compensating for dot gain ina stochastic screen. An image 40 represents a portion of a stochasticscreen at a 50% grey scale value. An image 42 represents a portion of astochastic screen at a 50% grey scale value with a number of dotsremoved to compensate for a 20% dot gain. As can be seen by viewing thetwo images 42 and 40, some of the dots in the image 40 are removed fromthe image 42 and the pattern of dots has changed. The removal of dotsand the modification in their position results in a loss of informationand detail in the image that is eventually printed.

In contrast to the conventional method of compensating for dot gain byremoval of information from the image, the present invention increasesthe size of the dots in the stochastic screen files and then reduces thesize of the dots to compensate for dot gain. To increase the size of thedots in the stochastic screen files, the present invention screens thecontinuous tone CMYK files without first increasing their resolution toequal the resolution of the output device. Once the files are screened,then the files are expanded to equal the resolution of the outputdevice. Expanding the screened files effectively makes each dot in thestochastic screen file larger. Subdots, i.e., portions of these largerdots can then be selectively removed to compensate for dot gain. Theresult of the subdot shaving is a stochastic screen with dots of unevensizes and shapes.

FIG. 3 illustrates a portion 50 of a stochastic screen file at arelatively low, e.g., 300×300 dpi resolution. If the file were expandedto a 1,200×1,200 resolution, then each dot in the portion 50 would beexpanded to be 16 “subdots” in the expanded file. With 16 subdots in theexpanded file representing a single dot in the lower resolutionstochastic screen file, certain subdots 52 can be removed from theexpanded file to compensate for dot gain. The particular subdots whichare removed can be selected by a user through a computer interface whichallows a user to select which subdots are removed. In the presentlypreferred embodiment of the invention, subdots are generally removedfrom the periphery of the cluster of subdots to compensate for theexpansion of the ink that occurs with dot gain. Because the removal ofsome subdots from the expanded stochastic screen file does notcorrespond to the removal of an entire dot in the lower resolutionstochastic screen file, the resulting images do not exhibit acorresponding loss of detail.

In the example shown in FIG. 3, the same subdots are removed from eachcluster of subdots. However, this is not necessarily required. FIG. 3Aillustrates a portion of a lower resolution stochastically screened file54. The file can be expanded by four times to create an expandedstochastically screened file 56. In the expanded stochastically screenedfiled 56, each dot in the file 54 is represented as a cluster of sixteensubdots. A mask 58 that is larger than each 4×4 cluster can be defined(in this case the mask is 8×8)having certain subdots removed (asindicated by the black dots). The mask is laid over the expandedstochastically screened file 56 to determine which subdots in theexpanded stochastically screened file should be removed.

The particular subdots in the mask that are removed in the mask may befixed or be random. The use of a mask that is larger than the clustersof subdots allows for a more accurate compensation for dot gain. Not allthe dots in the expanded stochastically screened file need to becompensated in the same way. Similarly, different patterns or number ofsubdots may be removed from the different color screens.

FIG. 4 illustrates a flow chart of the steps performed by the presentinvention to compensate for dot gain. Beginning at a step 60, thecontinuous tone CMYK files are created by a scanner or received in acomputer-readable form such as from a disk or over a computer network.At a step 62, the continuous tone files are stochastically screened at aresolution that is less than the resolution of an output device such asa printer, image setter, or other device that will be used to print theimages. At a step 64, the lower resolution stochastic screen files areexpanded to have a resolution that is equal to the resolution of theoutput device.

At a step 66, a test image is printed and a densitometer is used tomeasure the dot gain of the printed image. Once the dot gain has beencomputed, the expanded stochastic screen files are modified to removecertain subdots to compensate for dot gain at a step 68.

The particular subdots removed from the expanded stochastic screen maybe determined with a look-up table based on past experiences with priorprinting jobs. Alternatively, certain subdots can be removed and anothertest image printed to determine if the dot gain has been properlycompensated.

Another benefit of the present invention is the ability to control wheninks in a printing process are deposited directly over each other. Inmany printing jobs, such as printing on metal or on absorbent fabric,the deposit of two different colored inks directly over each othercreates a muddy or more otherwise blurred image. Therefore, the presentinvention can analyze the stochastic screen files to determine when dotsof different colors are deposited directly over each other. Asillustrated in FIG. 5, to prevent the blurring, the present inventionturns off those overlying dotes and replaces them with a dot in anotherstochastic screen file that will deposit a color equal to the colorcombination produced by the previously overlapping dots. For example, ifa dot 80 in a cyan file overlaps with a dot 82 in a magenta file, theresulting combination should be blue. Therefore, both dots can beremoved from the stochastic screen files and an additional screencreated have a dot 84 that will deposit blue ink at the location of thepreviously overlapping dots. When the dots are expanded to have aresolution equal to the resolution of the output device that will printthe image, subdots in the expanded files can be removed to compensatefor dot gain. To prevent colors from running together, at least one fullrow and one full column of the subdots at the periphery of each clustershould be removed.

FIG. 6 illustrates one common stochastic screen pattern used in gravureand flexography printing. The pattern 100 is a diamond shape 102 that issurrounded by 45° clear areas of grooves 104. For gravure, both the sizeof the center diamond and the size of the 45° grooves are equallyimportant and independent of each other. In the example shown, thecenter diamond had a width of 5 dots and has a groove that is 3 dotswide. The size could be varied such as having a center diamond that is15 dots wide surrounded by a groove that is 2 dots wide depending on theresolution desired. Individual subdots of center diamonds can beselectively removed to compensate for the dot gain that occurs in theprinting process.

Another benefit of the present invention is to perform color calibrationfor example, if the expected color, in the CMYK color gamut, of thecombination of 100% yellow and 100% magenta should be the color red, andif, instead the resulting color at printing that combination is thecolor orange, then a method for color correction is required. A methodfor compensating the color according to the present invention is toreduce the amount of yellow color by removing some of the subdots of thehigher resolution stochastically screened yellow file. Therefore thecolor combination will print the correct color red.

The changes for color compensation are for particular areas of theimage, so the entire image has to be analyzed in small portions of thestochastic combinations; unless whole areas of the image are of the samecombination CMYK colors, as in the case of spot color simulation throughCMYK color combination.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method of compensating for dot gain in stochastic screen files tobe printed on an output device having a predefined resolutioncomprising: receiving continuous tone CMYK files at a resolution lessthan the resolution of the output device; screening the continuous toneCMYK files to produce stochastic CMYK files at a resolution less thanthe resolution of the output device; expanding the stochastic CMYK filesto have a resolution that is equal to the resolution of the outputdevice wherein the expansion causes each dot in the stochastic CMYK fileto be represented at a number of subdots in the expanded stochastic CMYKfile; and selectively removing a number of the subdots in the expandedstochastic CMYK file to compensate for dot gain when the expandedstochastic CMYK files are printed.
 2. The method of claim 1, wherein theoutput device is a printer.
 3. The method of claim 1, wherein the outputdevice is an image setter or plate setter that produces films or platesof the expanded stochastic CMYK files.
 4. The method of claim 1, whereinthe CMYK files are screened as a number of diamond shaped dots that aresurrounded by grooves at 45°, wherein the diamond shaped dots and thegrooves are expanded prior to the removal of selected subdots.
 5. Amethod of preventing color overlap in stochastic screen printing,comprising: receiving continuous tone CMYK files; screening thecontinuous tone CMYK files to produce stochastic CMYK screen files at aresolution less than the resolution of an output device; determining ifthere are overlapping dots in the stochastic CMYK screen files and ifso, removing the overlapping dots and adding one or more additionalstochastic screen files having dots at the position of the overlappingdotes; and printing colors with the additional stochastic screen filethat represent the color of overlapping dots.
 6. The method of claim 4,wherein the stochastic screen files are expanded to create a number ofsubdots for each dot in the stochastic screen files and selectivelyremoving subdots in the expanded stochastic screen file to compensatefor dot gain.
 7. The method of claim 6, wherein the subdots are removedfrom a row and column of a cluster of subdots that correspond to asingle dot in the lower resolution stochastic screen file to preventcolors from running together.
 8. A method of correcting color inprinting the utilizes stochastic screens comprising: receivingcontinuous tone CMYK files at a resolution less than a resolution of anoutput device; screening the continuous tone CMYK files to producestochastic CMYK files; expanding the stochastic CMYK files to have aresolution of the output device, wherein the expansion causes each dotin the lower resolution stochastic screen file to be represented as anumber of subdots in the expanded stochastic CMYK file; and selectivelyremoving-subdots from the expanded stochastic CMYK file to correct thecolor when the expanded stochastic CMYK files are printed.